Many of polyurethane resin coating materials are mainly composed of polyurethanes obtained by reacting aliphatic polyester polyols or polyether polyols with organic polyvalent isocyanates, and resins having low glass transition temperatures and large elongation are generally used.
However, recently, demands for coating materials, coating compositions and the like which have high glass transition temperatures and can provide tough coats have been remarkably increased in various fields.
For example, a resin which has a high glass transition temperature, excellent heat resistance and high solubility in solvents as well as a low oligomer content is desired for a back coat layer of a heat sensitive ribbon tape for computers, word processors and the like, a dye-receptor layer or binder or sublimation type heat sensitive recording chart for video color printers, a binder or back coat layer of magnetic recording mediums and the like.
A magnetic tape or floppy disc which is widely used as a recording medium is produced by dispersing acicular magnetic particles having major axes of not more than 1.mu. together with additives such as dispersing agents, lubricants, antistatic agents and the like into a solution of a binder to make a magnetic coating composition and then applying the resulting composition to a polyethylene terephthalate film to form a magnetic layer.
Examples of the properties required for the binder of the magnetic layer include dispersion properties, filling properties and orientation properties of magnetic particles; durability, wear resistance and heat resistance of the magnetic layer; adhesion properties with a non-magnetic base; and the like, and the binder plays an extremely important role.
Further, a back coat layer provided at the opposite side of the magnetic layer on a non-magnetic base influences on traveling properties of a magnetic tape. Examples of the properties required for a binder of the back coated layer include durability, wear resistance, heat resistance and adhesion properties with the non-magnetic base, and the binder of the back coated layer also plays a extremely important role.
As the binder of the magentic layer or the back coat layer, a mixture of an adipate type or polycaprolactone type polyurethane resin and nitrocellulose or vinyl chloride polymer has mainly been used heretofore.
In a magnetic recording medium, magnetic particles are more micronized and highly filled and oriented in a magnetic layer and the surface of the layer is smoothed to improve s/n ratio (ratio of signal to noise) and to increase memory density. Further, a back coat layer is also smoothed to prevent lowering of the output of a magentic tape due to transfer of unevenness of the back coat layer to a magnetic layer in the case of storing the tape in a rolled state. As the surface of a magnetic layer or back coat layer becomes more smooth, traveling properties and traveling durability of the magnetic tape become more inferior and, therefore, a binder having good durability, wear resistance, heat resistance and adhesion properties with a non-magnetic base has been desired. Conventional binders for magnetic layers or back coat layers are insufficient with respect to these demands.